Design & Drafting Services
NOTE: These symbol illustrations and definitions are not intended for use as wiring guidelines. They are included to aid in interpretation of electrical schematic symbols. No warranty is expressed or implied as to the suitability of this information for any purpose.
Typical Generic Symbol for Relay With Two Contacts
A relay has electrical switch contacts that are operated by a magnetic field produced by an electromagnet. When the relay coil (electromagnet) is energized by applying electrical power, the resulting magnetic field moves a lever which moves electrical switch contacts--open contacts are closed and closed contacts are opened. Relays may have any number of contacts. The number of normally open contacts may differ from the number of normally closed contacts. All contacts on a relay usually have the same electrical characteristics--current carrying capacity.
Typical Generic Symbol for Contactor With Three Power Contacts & One Auxiliary Contact
A contactor is a type of relay designed for high-current applications. Like relays, contactors may have any number of contacts and combinations of normally opened and normally closed contacts. However, contactors usually have auxiliary contacts for use in low current control logic circuits and power contacts to switch high currents. Power contacts are larger and offer higher current carrying capacity than auxiliary contacts.
Typical Generic Symbol for Motor Starter With Three Power Contacts & One Auxiliary Contact
A motor starter is a contactor with overloads to protect motors from burn-out. Current flowing to the motor first passes through overload heaters. Excessive motor loading increases current through the heaters--increases overload heating. The overload contacts react to this heat and open the control circuit. Without a control signal, the contactor returns to its de-energized state interrupting current flow to the motor. Many manufacturers use the same housing for their contactors and motor starters. The overload unit is simply left off of the contactor. Motor starters are usually shipped with the overload unit installed, but the overload heaters must be ordered separately to match the motor.
Typical Generic Symbol for Overloads
For an explanation of overload operation, see the motor starter symbol.